A pretty good definition at Wiki: "It can be a special repetition, arrangement or omission of words with literal meaning, or a phrase with a specialized meaning not based on the literal meaning of the words."
In other words, figures of speech add poetry and color to language. When over-used, they lose their power and fade into ordinary and become, really, unheard.
If you had OCD you could have been a Renaissance scholar classifying the 184 types of figures of speech, but the five most of us learned in grade school are simile, metaphor, hyperbole, personification and synecdoche.